If you're flirting with a good friend of yours and you like him. So basically just flirting to see if he likes you back. Most of the time, he doesn't say something back but it seems to like it. He'll like breathe faster, look away and smile. Is it a normal, polite response or an indication that he likes me back?

Also, he probably has low self esteem although it's not obvious if you don't know him well enough. He can break the ice very well, usually when he steps in, he'd be talking to people and is so confident, which is what I
like about him.

But if he calls someone and they don't pick up and don't call back, he'll start imagining that that person doesn't like talking to him or maybe there is some misunderstanding. I'd tell him that they could be busy or might be too tired and so on...Happens right?

Basically, very sensitive person, touchy about issues like not fitting in.
When people suggest that we should consider getting together, he'd tell them jokingly, "why would she want someone like me?"
He's confident about his personality but not his looks. He thinks he's fat and ugly and not good enough for any girl. Frankly, he is plump but I don't think he's ugly 'cos he's got nice eyes.

I don't know how to tell someone like him that I like him when he believes he's not good enough for a girl. He thinks I'm pretty, would say it indirectly like ask his friend, "do you think she's pretty?" They'd nod and he'll say "he thinks you're pretty, you happy now?" jokingly. If I tell him I like him, I don't want him to think it's pity. Like, "poor friend, he can't get a gf, I should say I like him." Please help.

I am exactly like your friend. Almost down to a tee. sort of eerie really.....

but anyway my advice to ask him out. If hes as like me as i think he is he'll be too shy to actually ask anyone out himself and would be delighted to be asked out. Go for it, if hes got low self esteem hes unlikely to say no anyway.